1. Field of the Invention
This invention relates to electrophoretic displays having improved performance.
2. Brief Description of Related Art
The electrophoretic display (EPD) is a non-emissive device based on the electrophoresis phenomenon of charged pigment particles suspended in a dielectric solvent. It was first proposed in 1969. The display usually comprises two plates with electrodes placed opposing each other and separated by spacers. One of the electrodes is usually transparent. A suspension composed of a colored solvent and charged pigment particles is enclosed between the two plates. When a voltage difference is imposed between the two electrodes, the pigment particles migrate to one side and then either the color of the pigment or the color of the solvent can be seen according to the polarity of the voltage difference.
There are several different types of EPDs. In the partition type EPD (see M. A. Hopper and V. Novotny, IEEE Trans. Electr. Dev., 26(8):1148-1152 (1979)), there are partitions between the two electrodes for dividing the space into smaller cells in order to prevent undesired movement of particles such as sedimentation. The microcapsule type EPD (as described in U.S. Pat. Nos. 5,961,804 and 5,930,026) has a substantially two dimensional arrangement of microcapsules each having therein an electrophoretic composition of a dielectric solvent and a suspension of charged pigment particles that visually contrast with the dielectric solvent. Another type of EPD (see U.S. Pat. No. 3,612,758) has electrophoretic cells that are formed from parallel line reservoirs. The channel-like electrophoretic cells are covered with, and in electrical contact with, transparent conductors. A layer of transparent glass from which side the panel is viewed overlies the transparent conductors.
An improved EPD technology was disclosed in co-pending applications, U.S. Ser. No. 09/518,488 filed on Mar. 3, 2000 (corresponding to WO01/67170), U.S. Ser. No. 09/606,654 filed on Jun. 28, 2000 (corresponding to WO02/01280) and U.S. Ser. No. 09/784,972 filed on Feb. 15, 2001 (corresponding to WO02/65215). The improved EPD comprises closed cells formed from microcups of well-defined shape, size and aspect ratio and filled with charged pigment particles dispersed in a dielectric solvent.
As in liquid crystal and other displays, an EPD may be a segment display, a passive matrix display or an active matrix display, depending on the driving mechanism and the circuitry design. The passive matrix driving system is one of the most cost effective driving mechanisms. The system has row electrodes on the top side and column electrodes on the bottom side, of the cells. In most cases, the top row electrodes and the bottom column electrodes are perpendicular to each other. Generally speaking, a threshold voltage of no less than ⅓ of the driving voltage is required to suppress or eliminate the undesirable cross-bias or crosstalk effect in adjacent pixels of a passive matrix display.
Crosstalk occurs when the particles in a cell are biased by the electric field of a neighboring cell. FIG. 1 provides an example to illustrate crosstalk. A and B are two cells of a passive matrix EPD with a voltage bias of 30V and 0V, respectively. The bias voltage of the cell A drives the positively charged particles towards the bottom of the cell. Since cell B has no voltage bias, the positively charged particles in cell B are expected to remain at the top of the cell. However, if the two cells, A and B, are close to each other, the top electrode voltage of cell B (+30V) and the bottom electrode voltage of cell A (0V) create a crosstalk electric field which may force some of the particles in cell B to move downwards. Widening the distance between adjacent cells may eliminate such a problem; but the distance may also reduce the resolution of the display.
Alternatively, the crosstalk problem can be lessened if a cell has a significantly high threshold voltage. A large gamma (or a steep slope) of the response-voltage characteristic curve is also desirable to increase the resolution of a passive matrix device. However, cells in EPDs made using the electrophoretic materials and techniques currently available typically do not have the required response-voltage characteristics to prevent the undesirable movement of particles. As a result, the EPDs constructed from these materials and techniques usually cannot achieve high resolution.
Cross bias is another well-known problem associated with a passive matrix display. The voltage applied to a column electrode not only provides the driving bias for the cells in the scanning row, but it also affects the bias across the non-scanning cells in the same column. This undesired bias may force the particles of non-scanning cells to migrate to the opposite electrode. This undesirable particle migration causes visible optical density change and reduces the contrast ratio of the display.
In addition, in order to scan through all rows of electrodes in a frame within a reasonable time scale, a fast response rate is also highly desirable. However, most of the EPDs currently available have not shown an acceptable threshold characteristics or response rate required.
Electrophoretic fluids having inherent threshold characteristics have been reported by, for example, I. Ota, et al, in SID Proceedings, 18, 243 (1977) and Evans, et al, in U.S. Pat. No. 3,612,758. The fluids have reportedly shown disadvantages in response time, operation voltage, brightness, image uniformity or display longevity. In most cases, the fluid is in direct contact with the electrode conductor. The direct contact may enhance the particle-electrode interaction and, in some cases, result in a threshold, but with trade-offs in image uniformity and display longevity probably due to the irreversible adsorption and/or redox reaction(s) at the electrode surface.
A system having gating electrodes was disclosed in U.S. Pat. Nos. 4,655,897 and 5,177,476 (assigned to CopyTele, Inc.) to provide EPDs capable of high resolution at a relative high driving voltage using a two layer electrode structure, one of which layers serves as a gating electrode. Although these documents teach how the threshold voltage may be raised by the use of gating electrodes, the cost for fabricating the two electrode layers is extremely high due to the complexity of the structure and the low yield rate. In addition, in this type of EPDs, the electrodes are exposed to the solvent, which could result in an undesired electroplating effect. Electrophoretic displays comprising an in-plane gating electrode or a holding electrode to reduce or eliminate the cross bias and crosstalk of a passive matrix EPD have also been disclosed in copending patent applications, U.S. Ser. No. 10/242,335 filed on Sep. 11, 2002 (WO03/23510) and U.S. Ser. No. 10/282,444 filed on Oct. 28, 2002 (WO03/38512), respectively. However, the cost associated with the additional electrode is still an issue. Alternatively, magnetic particles and a magnetic electrode have been disclosed in U.S. Pat. No. 6,239,896 (assigned to Canon) to provide the required threshold, but also at the expense of manufacturing cost.
Therefore, there is a need for a cost effective method for inducing or enhancing the threshold voltage with a sharp gamma and high switching rate, without the trade-offs in image uniformity and display longevity, and without the need of complex circuitry designs.
The whole content of each document referred to in this application is incorporated by reference into this application in its entirety.